This invention relates to improvements in gas treatment, in particular to improvements in the removal of pollutants from hot gasses used to heat waste-heat boilers for power production.
In power generation using hot gasses to heat waste-heat boilers to produce steam, it is essential to filter particulates from, and treat acid in, the hot gasses prior to discharge downstream of the boiler.
Generally, after combustion, polluted hot exhaust gasses are passed over/through a waste-heat boiler to extract the heat therefrom to produce steam which powers steam driven turbines. Post boiler, when the polluted gasses have cooled they are passed through a combined low temperature filtration system to remove particulates and an acid treatment system to neutralize the acids in the polluted gases. Furthermore, activated carbon is generally used downstream of the boiler as part of the pollutant control system.
The result of the above are that:                1—The gases entering the boiler is filled with pollutants that get deposited on the waste-heat boiler tubes (heat exchangers)        2—For some processes that create hot gas from waste processing (municipal solid waste (MSW) and demolition waste) sticky ash (which is a pollutant that becomes fluid at high temperatures and get suspended in the hot gases and transported into the boiler) impinges on the boiler tubes, cools down and gets attached to the waste-heat boiler tubes forming a solid coating thereon that reduces the boilers tube lifetime, reduces heat exchange efficiency, and forces boiler shutdown for maintenance and/or cleaning. Known industry solutions involve reducing the temperature of the hot gas to below 600° C. so as to cool the sticky ash prior to the boiler so that it is no longer sticky. Lowering the temperature prior to the heat exchange tubes of the boiler reduces the efficiency of the system. Another known solution is to use a boiler with heat exchange tubes in parallel with the gas flow (known as a superheating part of the boiler) to remove heat from the gas flow before the gas impinges on the perpendicular tubes of the boiler. The hot gases, as not impinging on the parallel tubes, can give up some of their heat in the superheating part without the sticky ash binding to the boiler tubes. While this improves the efficiency by using more of the heat it is a far from perfect solution and some sticky ash always finds its way to the tubes and sticks to them resulting in improved efficiency but only slowing the problem rather than solving it.        3—Tar is typically generated as a result of the low temperature gasification of organics in the feedstock or incomplete combustion of the organics. The tar is carried from the processing chamber, where it is created, suspended in the hot gases into the thermal oxidizer. Typically the tars disassociate at elevated temperatures (as the case in the well designed and operated thermal oxidizer) into a simpler smaller molecules of Carbon and Hydrogen (CO, CO2, H2, H2O, etc.). However, combustion system upsets and system transitional periods could, on occasion, lead to tar passing out of the thermal oxidizer without becoming fully disassociated. These tars, if not destroyed in the thermal oxidizer, impinge on the filters and bind thereto and, with time, render them inoperable as gas filters. The current industry solution is to wait for the hot gases to cool down and to collect the solidified carbons from the Tar at the cold filter waste collection.        4—Since the acid is neutralized after the boiler, the boiler heat exchange pipes are subject to high temperature acid attack which limits the life of the boiler.        5- The existent of acid also forces the boilers to operate at a reduced efficiency, by insuring that the heat extracted from the hot gas is only partial so as to ensure that the boiler exit temperature of the gases is above the dew point of acid formation.        
All the above results in the necessity to regularly clean inside boilers, in particular the heat exchange tubes which regularly requires the boiler to be taken off line.
Many sources of hot gas can be used for power generation including, but not limited to, combustion of fossil fuel, e.g. coal, natural gas etc; combustion of municipal waste or demolition waste; and by product gas of other processes, e.g. oxidized or combusted volatile organic compounds from waste treatment processes. Waste heat capture for energy recovery is now becoming more widely used on many industrial processes of a varying scale.
In waste heat capture it is generally considered to be good practice to maintain, as far as is possible, unhindered gas flow upstream of, and through, the boilers.
It has been proposed previously to remove acid from hot gasses by the introduction of lime upstream of a boiler, for example it is known from GB2160301 to inject lime into combustion gas and then pass it through a particulate removal tower containing granulated slag prior to the gas exiting and passing through a boiler. A further example of use of lime upstream of a boiler is disclosed in JP 11276851 which discloses a similar type system but does not clearly describe the means of removing the lime from the gas.
None of the above systems address problems associated with sticky ash or tar and in all of the above systems the cleaning of the boiler and/or filters require that the boiler be taken off line. This is a significant disadvantage as having to take a boiler offline results in reduced efficiency and having to take filters or gas cleaning off line results in one of the necessity to shut down the gas flow or the discharge of un-cleaned gas either directly into the boiler or into the atmosphere or both.
A further reason that filters are generally not placed upstream of the boiler, in particular with a system that comprises a thermal oxidiser, is that thermal oxidizers typically operate at a fixed pressure set point (within a small window range). If known filters are directly downstream of the thermal oxidiser then as they become blocked over time the pressure would build-up gradually in the thermal oxidizer. Furthermore, if lime is used then dosing of reagent lime will lead automatically to pressure buildup in the thermal oxidizer.
The reduced flow and associated back pressure on the thermal oxidiser would impact upon the operation of the pressure sensitive burners in the oxidiser and would potentially extinguish the burner, and force the thermal oxidizer into a mal-operation, or even a dangerous operation. Hence, for these reasons alone typically filters are not used between a thermal oxidiser and the waste heat boilers.
It is an object of the present invention to provide an improved and robust means of cleaning hot gas, upstream of a boiler that addresses the problems associated with current technologies.